Cell surface carbohydrates change during development suggesting that they are probably involved in differentiation. These developmentally-regulated changes allow some antibodies directed against carbohydrates to discriminate among tissues, both normal and malignant, and at various stages of development. To obtain cell-specific monoclonal antibodies, mice and rats have been immunized with various cell types in many laboratories. Some of the antibodies derived from hybridized spleen cells of the immunized animals that have an apparent specificity for certain cells and developmental stages are directed against carbohydrates. We have elucidated the structure of over 100 of these carbohydrate antigens. The antibodies are being used to study changes in cell surface carbohydrates during development in hopes of providing insights into the functions of glycoconjugates. Some are also being evaluated for their possible usefulness in serum tests for cystic fibrosis and various cancers. For example, antibody CC3C195 detects elevated levels of mucin in serum from many patients with cancer of the colon and pancreas, or with cystic fibrosis. The antibody binds to the sialylated human Le(a) blood group antigen like the previously reported antibody 19.9. However, unlike 19.9, CC3C195 also binds to the Le(a) antigen itself. This broader specificity may make CC3C195 more useful for diagnosis than 19.9.